If the current is always adjusted to a definite value the slide wire may be graduated to read millivolts or temperature directly. Various devices are employed for adjusting the current to the proper value. An ammeter will serve the purpose but the usual method is to obtain a preliminary setting by replacing the thermocouple by a constant known source of electromotive force such as a standard cell. The galvanometer G is always used as a zero instrument in a strictly potentiometric circuit. Hence, it requires no calibrated scale and no attention need be given to the constancy of its sensitivity provided it is always sufficiently sensitive to serve its purpose. These requirements are easily met, and the entire potentiometer, galvanometer, battery, standard cell, slide wires, etc., as constructed are mounted in a case not much larger than that of a millivoltmeter. The instrument is as mechanically robust as many indicators operating on the galvanometric principle.
There are several important advantages in the potentiometer method. The electromotive force or temperature scale is easily made very open thus permitting accurate readings. Scale lengths from 15 cm. to a meter and more are employed. The calibration of the scale is in no way dependent upon the constancy of magnets, springs, or jewel settings nor upon the level of the instrument. From the pyrometric standpoint, however, the greatest advantage is the complete elimination of any error due to changes in the resistance of the ciouple or of the lead wires. No matter what resistance is inserted in the thermocouple circuit, the reading of the potentiometer still gives the true electromotive force of the couple. The objections to the potentiometer are slightly greater initial cost and the fact that usually a manual adjustment must be made to obtain a setting. In the potentiometric recording instrument however all the various manipulations may be performed mechanically, even to the balancing against the standard cell, as in the recording potentiometer manufactured by Leeds & North-rup Co.
Semi-potentiometer Method.—It is possible by means of a single galvanometer or millivoltmeter to measure the electromotive force of a couple potentio-metrically. Thus in Fig. 6 by using a shunted galvanometer first in the main circuit abcB, as an ammeter, the initial setting for a standard current is obtained. Then the instrument without the shunt is placed in the position G and the contact 6 is moved along the graduated slide wire until a zero setting is obtained. The objection to this method is that if the millivoltmeter is sufficiently sensitive to be used as a zero instrument it is not likely to be reliable as an ammeter, and if reliable as an ammeter it is not likely to possess sufficient sensitivity when used as a detector or null instrument. Various modifications of this device, however, have proven of value in thermoelectric pyrometry.
Northrup Pyrovolter.—Figure 7 shows the wiring diagram of the Northrup pyrovolter manufactured by the Pyrolectric Instrument Co. Referring to Fig. 7 (a), the dry cell B contained in the case of the instrument sends a current through the variable resistance R and the fixed resistances C and $. The resistance C of copper is equal in value to the resistance of the copper coil of the moving element of the galvanometer. The couple T is connected in series with the moving coil of the galvanometer across the resistance S. The resistance R is adjusted until the galvanometer reads zero by turning the knurled head on the lower right-hand corner of the instrument. The key on the lower left-hand corner is then depressed which throws the thermocouple and the resistance C out of the circuit and replaces C by the galvanometer G of equivalent resistance, Fig. 7 (b). The galvanometer is now deflected by an amount proportional to the current flowing through it, which in turn is proportional to the potential drop